Ballistic modifiers for double base propellants are used to modify burning characteristics of double base propellants. A double base propellant generally contains an energetic polymer, such as nitrocellulose, plasticized into a gel by an energetic plasticizer, such as nitroglycerine. Additives may be included in these double base propellants to improve the mechanical or ballistic properties of the propellant. One such additive is termed a ballistic modifier, which alters the inherently high dependence of the burning rate on chamber temperature and chamber pressure.
Ballistic modification allows the rocket motor to operate within a wider pressure range with only small changes in its burn rate, commonly referred to as a plateau. This phenomenon was first observed, accidentally, with the use of lead stearate, which was commonly used as an extrusion lubricant in solventless double-base propellants produced during World War II. The plateau is the result of a catalytic or super-rate effect on burn-rate that occurs at pressures well below the maximum burn-rate that is observed on unmodified propellant, thus producing burn rate curves with a slope approaching zero in the plateau region. When the slope of the burning rate curve becomes less than zero, the burning rate decreases with increasing pressure and results in what is termed a mesa. However, little has been known about the specific properties of the ballistic modifiers that affect propellant burning rates. The objective in ballistic modification of double base rocket propellants is to obtain plateau or mesa burning over a desired range of pressure and burning rate levels. These terms come from the shape of a log-log plot of the burning rate equation for double-base propellants which is given as: r=CPn or log r=n log P+log C, where r is the burning rate, P is the combustion chamber pressure, C is a constant for each propellant composition at any one temperature, and n is a constant for non-modified propellants but is a variable in modified propellants. In plateau- or mesa-burning propellants, “n” varies from very high positive values to zero or low negative values. Thus, a plot of log r against log P would give a straight line with a slope of “n” for a non-modified propellant, but a “plateau” shaped line or a mesa-shaped line for modified propellants. The performance of a ballistic modifier is measured in terms of the rate increase and pressure extent of plateau burning. Super rate defines the concept of substantially increasing burn-rate at any given pressure over burn-rates obtained from non-modified propellants.
There is a need in the art for improvements in ballistic modifiers for double base propellants. The present invention addresses this need and other needs.